[meteorite-list] Ordinary chondrites - rarest to the most common classes

[Jeff Grossman]

First off, be careful of the words "clan" and "class."  The way most of 
us use it, carbonaceous chondrites are a "class", comprising many 
groups.  Some C chondrite groups are associated into "clans", like the 
CV-CK clan or the CM-CO clan.  But not all people use the term clan, and 
those who do sometimes differ in which groups are in which clans.  In 
any case, a "clan" is an association of groups that are thought to be 
related in some way, not necessarily originating on the same parent body.

There are two major characteristics that separate ordinary from 
carbonaceous chondrites.  1) OCs are above the terrestrial fractionation 
line (TFL) for oxygen isotopes and CCs are below it (except for highly 
altered ones).  2) OCs are depleted in refractory lithophile elements 
and CCs either have solar abundances or above.  (E chondrites are on the 
TFL and depleted in refractories.)

R chondrites share these major properties with ordinary chondrites, and 
therefore are better lumped with the OCs.  In fact, bulk composition and 
O isotopes are the key properties used to assign all the C chondrite 
groups to the carbonaceous class.  I think it is reasonable to do the 
same with the R, H, L, and LL groups, that is, to assign them to a 
common class.  We don't really have a name for this class, as "ordinary 
chondrites" have come to be synonymous with H-L-LL, which I would 
consider to be a clan of this unnamed class.  I would NOT put R 
chondrites in the H-L-LL clan; they are in the same class. 

I guess part of the confusion is whether "ordinary chondrites" has to 
apply only to H-L-LL chondrites, or whether we can use this phrase as 
the name of an entire class.  My initial preference was to do the 
latter.  I said the ordinary chondrite class has two major clans, the 
H-L-LL clan and the R clan (which has but one member).  On reflection, a 
more palatable solution would be to find a new name for this class, and 
then we could refer to the ordinary chondrite clan and the R chondrite 
clan within it. 

So, what do we call this class?  You can't use "noncarbonaceous 
chondrites" because we also have the enstatite chondrites in their own 
class.  I have no idea.

Jeff


Carl 's wrote:
> Hi Jeff,
>
> I've been puzzled about what you said and perhaps I've misread or missed your comments. Why do you think the R chondrites should be included in the oc clan (rather than the carbonaceous)? I thought this was a very unique idea.
>
>
> Thank you all for this interesting topic.
>
> Carl
>
>
>
> Jeff Grossman wrote:
>
>   
>> I didn't say they ARE included in the OCs... I 
>>     
> said that I thought they should be. As far as I 
> know, I am alone in this opinion...
>
> and 
>
>   
>> ...If we take a more expansive definition of "ordinary chondrite" than most of my rather
>>     
> conservative colleagues are normally willing to accept, I would say that
> the rarest group of OCs is the R chondrites (only ~100 are known and
> many of those are paired).In addition, a number of unique ungrouped
> meteorites are OC-like.But again, I don't know of any colleagues who
> agree with me that R chondrites are in the OC class. [I would say that
> the OC class has two clans, the H-L-LL clan and the R clan].
>
>
>
>  		 	   		  
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-- 
Dr. Jeffrey N. Grossman       phone: (703) 648-6184
US Geological Survey          fax:   (703) 648-6383
954 National Center
Reston, VA 20192, USA